Gyros have been used to measure rotation rates or changes in angular velocity about an axis. A basic conventional fiber optic gyro (FOG) includes a light source, a beam generating device, and a coil of optical fiber coupled to the beam generating device that encircles an area. The beam generating device transmits light beams into the coil that propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction along the core of the optical fiber. Many FOGs utilize glass-based optical fibers that conduct light along a solid core of the fiber. The two counter-propagating (CW and CCW) beams experience different pathlengths while propagating around a rotating path, and the difference in the two pathlengths is proportional to the rotational rate.
In general, the size of the FOG affects the accuracy or sensitivity of a FOG. For example, smaller FOGs typically have less accuracy than larger FOGs. FOGs have accuracies that generally increase with the area encircled by the optical path of the light beams. Thus, the larger the area enclosed by the optical path, the greater the signal-to-noise ratio of the FOG. Also, to improve the signal-to-noise ratio of the FOG, the optical path may be increased by increasing the number of turns of the coil.
In a resonator fiber optic gyro (RFOG), the counter-propagating light beams are monochromatic and recirculate through multiple turns of the coil and for multiple passes through the coil using a recirculator such as a fiber coupler. The beam generating device typically modulates and/or shifts the frequencies of each of the counter-propagating light beams so that the resonance frequencies of the resonant coil may be observed. The resonance frequencies for each of the CW and CCW paths through the coil are based on a constructive interference of successively recirculated beams in each optical path. A rotation of the coil produces a shift in the respective resonance frequencies of the resonant coil and the frequency difference associated with tuning the CW beam and CCW beam frequencies to match the coil's resonance frequency shift due to rotation indicates the rotation rate. In the RFOG, effects stemming from the presence of glass material of the optical fiber may shift the resonance frequencies of the CW and CCW paths and thus produce a false indication of rotation or inaccurate measurement of rotation rate. A reflective mirror may be used to recirculate the counter-propagating light beams in the coil but this typically reduces the signal-to-noise ratio from losses generated at the transition from the mirror to the coil. Anomalies that decrease the accuracy of the measurement of the rotational rate may be generated from a non-linear Kerr effect, stimulated Brillioun scattering, polarization errors, bend losses. These error mechanisms are also sensitive to the environment which, for example, gives rise to unwanted temperature sensitivity.
The non-linear Kerr effect occurs when high monochromatic light power inside the RFOG alters the index of refraction of the glass in the optical fiber. A mismatch of intensities of the CW and CCW beams may induce a bias on the observed frequency shifts on the order of several degrees/hour. Stimulated Brillioun scattering (SBS) occurs when a high intensity associated with a high finesse in the fiber resonator causes lasing in the glass fiber, and this lasing generally promotes large instabilities in the measurement of the resonance frequencies. Polarization-induced errors may result from fiber couplers that incidentally couples light into a second polarization mode, either from one optical fiber to an adjacent optical fiber or within the same fiber. Light may also be cross-coupled between polarization states in the fiber coil itself. The second polarization mode may resonate to produce an asymmetry in the resonance lineshape of the polarization mode used to measure a rotation. Even though the resonance frequency of the second polarization mode is the same for the CW and CCW beams, the amplitude may be different, thus causing different observations, beyond the affect of rotation, of the resonance frequencies of the CW and CCW beams. Polarization-induced error may severely limit the accuracy of the RFOG because determination of the resonance centers for each of the resonance frequencies of the CW and CCW beams directly affects the rotational rate measurement.
Accordingly, it is desirable to provide a fiber optic gyro capable of measuring rotational rates with an accuracy sufficient for navigation systems. In addition, it is desirable to provide a high accuracy fiber optic gyro for integration with relatively small platforms. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.